If two variables are normally distributed and are correlated in such a way that their scatter plot is football shaped. X values which are above average tend to have on average lower y values. This is seen whenever there is spread.
This is known as Regression effect. This cannot be attributed to any external cause other than ordinary variability.
Regression fallacy is attributing an external cause to the regression effect. For example lets say a new bie in cricket does extremely well and becomes a star overnight. He gets a lot of endorsements and good media coverage. However next year he does not perform equally better. His performance may even be below average. This could be due to pure variability. However, attributing it some external cause for example saying that all that success has gone into his head etc is wrong. This is regression fallacy.